So I'm a geek and into fanfiction for a few years now and I've come to form my own opinion on this subject, though I feel it might be a bit of an unpopular one: I actually don't find Mary Sues to be the scum of the Earth. In fact I don't really hate them that much. Don't get me wrong I find them really cringe worthy, but they're really not deserving of all the hate they get nor do the creators of said "abominations" deserve to be bullied for their characters because bullying actual people for really any reason makes you a lot worse than them; I mean c'mon all they did was make a crappy fan character. If you ask me, that doesn't really warrant ridicule. There has been at least one occasion where on this site I came across a person who actively makes fun of people's OCs saying that it's their right to do so (it was along those lines anyway) to which I refused to read past that point.
From my experience it's usually though not always the younger fans that end up making the worst of the Mary Sues, which is okay because well they're young and chances are they aren't as enthused about making well rounded and believable OCs as much as we are and choose instead to focus more on their fantasies, which is perfectly acceptable seeing as how it's called fanfiction for a reason.
I mean it's not like they're forcing you to read it. They just posted it to show their love for a series/character; now granted once it's posted that pretty much means that it's free game to criticism as long as it's helpful and not rude; most people in fact like it. It helps them learn and grow as a writer. Others hate it when you tell them that any part of their story is less than perfection and those people are, well I hate to say it, but they're delusional because really no piece of writing is perfect. There are always flaws and ways to improve your work that's just life.
I really hate the fandom community when it comes to the subject of what makes an OC a Mary Sue: the most universally known definition of a Mary Sue is a female character so perfect it's actually very annoying but according to some people, there are smaller scale things that just make a character a Mary Sue; if she has heterochromia she's a Mary Sue, if she's incredibly beautiful she's a Mary Sue, if she has amazing powers, insta-Sue, and so on and so forth.
Honestly no one ever talks about what kinda clothes a Sue wears or the canon characters she dates unless you're in the creepypasta fandom (which is weird, why do people wanna bone eldritch-esque horrors and murderers?). Nobody ever talks about the fact that most girl OCs are made to hate all things even remotely "girly" and not just OCs. I mean female characters in general seem to wear nothing but dark coloured hoodies, jeans and sneakers/combat boots and have like an allergy to dresses and skirts back in Quizilla nearly every female protagonist's dialogue pertaining to her appearance would be like:
"I put my hair up in a ponytail/messy bun and do my makeup; a little mascara and colorless lip gloss, I never wear a lot of makeup because I don't want to look like a streetwalker like all the other girls. I head over to my closet and pull out my dark grey hoodie sweater, black skinny jeans and my black and white converse because unlike other girls I seriously hate dresses and skirts and all things bright and colorful (Author's note: yeah I'm basing her style off mine)."
Oh wow so in order to be unlike any other girl, you wear exactly things other girls actually do wear? Talk about pretentious. I mostly wear black and/or white but unlike these so totally cool and original tomboy wannabes I am actually open to wearing other colours and I wear skirts and dresses (with shorts underneath mostly but still).
I'd like you all to bring your attention to the picture I used up top... no I don't know who made it but I'm using it to help prove my point; the "original character" seems like more of a Mary Sue than the "Mary Sue" does. She's "tough" as well as "rude/vulgar" and doesn't like anything even remotely feminine. She scoff at all the other girls and their love for makeup, boys, and all things cute; in fact she detests those prissy princesses. They haven't actually done anything terrible to her other than disagree with her or bully her a little.
Now don't get me wrong the girl on the Mary Sue side of the picture is what many think of when they hear the word but here's the thing: take away the sparkles and she looks just like any stereotypical high school-20 year old girl. Seriously I actually prefer the so-called Mary Sue example as the protagonist because I know of people who would look like her and with her speech balloon you could just make her personality ditsy and friendly; considering how many people want to make their female characters complete jerks because that makes them strong female characters, right? No it does not, like at all. I would honestly love the change if only to break up the monotony of crappy females in fiction.
If you ask me the defining traits of a Mary Sue should be: Is absolutely perfect, only flaw is not actually used as a flaw, everyone loves her despite not actually knowing her all that well, the villain either wants to capture her or is converted to good by her, and everyone hates whoever even so much as disagrees with her, every good thing happens to her despite her not actually doing anything to earn it, she is described or thought of absolutely flawless in her appearance despite looking very plain, and finally has the personality of a broom.
Everything I've listed off is just stuff that I've noticed in things like fanfiction, teen movies/books, and even some literature targeted towards older women, though it's mostly found in YA novels and is mainly used to make the main character less of a character and more of a reader insert or in some cases a writer insert which, let's be honest, is just another way of saying "this is not actually a character but rather a way that unimportant people (most often women) can feel like they are the masters of this world, have all the hot guys begging for their attention and are the only people capable of doing anything of importance." Putting it like that it kinda makes it sound like in most cases the reader is the ultimate Mary Sue but that's crazy, right?